The invention relates generally to conversion of incident microwave radiation to visible light. In particular, the invention uses electroluminescence to receive and detect microwave radiation and in response emit photons in the visible portion of the electromagnetic spectrum
Exposure to high-power microwave (HPM) radiation represents a concern for military personnel as a consequence of operational environments. Microwaves, broadly including ultra-high frequency (UHF) and extreme-high frequency (EHF) can be defined as electromagnetic radiation with wavelengths λ of approximately between 1 m and 1 mm (10−3 m), which correspond to frequencies v=c/λ between 0.3 GHz (109 Hz) and 0.3 THz (1012 Hz). This can be alternatively expressed as photon energies E=ℏω=hv=hc/λ between 1.24 μeV (10−6 eV) and 1.24 meV (10−3 eV), where h≈4.136 feV-s (10−15 eV-s)≈6.626×10−34 J-s is Planck's constant, ℏ=h/2π is Dirac's constant, and ω is angular frequency. Note that units m is meters, Hz is hertz (cycles-per-second), eV is electron-volts, J is joules, s is seconds, and prefixes P, T, G, M, k, m, μ, n, p, f and a correspond respectively to peta (1015), tera (1012), giga (109), mega (106), kilo (103), milli (10−3), micro (10−6), nano (10−9), pico (10−12), femto (10−15) and atto (10−18).
Humans lack natural ability to detect the presence of such radiation. By contrast, human eyes have sensitivity in the visible light portion of the electromagnetic spectrum. This visible portion ranges in wavelength from 0.75 μm (red) to 0.38 μm (violet), which corresponds to frequencies between 400 THz and 790 THz, expressible as energies between 1.7 eV and 3.3 eV. Consequently, to enable visible detection of HPM radiation would entail raising received photon energy by between three (3) and six (6) orders of magnitude.
Many types of luminescence are defined by the mechanism used to excite a luminescent material to induce radiation emission. Examples of luminescence include photoluminescence (photon absorption), radioluminescence (ionizing radiation), chemiluminescence (chemical reaction), and electroluminescence (electronic current). An artificial luminescent material or “phosphor” can be produced from generally powdered ceramic semiconductors with a crystalline material structure subtly altered by the introduction of dopant materials.
Photoluminescence constitutes a physical process that alters the wavelength (or “color”) of electromagnetic radiation. In the process of photoluminescence, electromagnetic radiation strikes a material that absorbs the incident energy (thereby elevating an electron in a ground state to a higher discretized energy shell) and re-releases that energy in the form electromagnetic radiation of a different wavelength (as the energized electron transitions to a lower discretized energy shell). Change to a longer wavelength (at lower energy) is called Stokes emission or down-conversion; and change to a shorter wavelength (at higher energy) is called anti-Stokes emission or up-conversion.
Conventional phosphors have been designed that can down-convert visible light into near-infrared, night-vision compatible radiation or near-infrared light to mid-infrared thermal-vision compatible radiation. Conventional phosphors have been designed that can up-convert near-infrared radiation into visible light, or red-orange color into green-blue color, or visible light into ultraviolet radiation in a wavelength band about 0.2 μm.
To produce the up-conversion effect and satisfy conservation of energy, up-conversion phosphors require multiple low-energy, long-wavelength photons to produce a single higher-energy, shorter-wavelength photon. Two-photon processes are common, and there are phosphors that can use three low energy photons to produce a single higher energy photon. The photonic nature of the up-conversion process imposes limits on the up-conversion wavelength change. The ratio of the wavelength of incident electromagnetic radiation to the wavelength of emitted electromagnetic radiation in up-converting phosphors is typically between 1.5 and 3.5.